Environmental Engineering Reference
In-Depth Information
to the next, a process called
genetic drift
. In small popu-
lations, the consequences of such random sampling
are much more pronounced than in large populations.
First, in small populations rare alleles are more easily
lost and common alleles are more easily fi xed by chance
than in larger populations, resulting in a loss of genetic
variation. Second, in small populations, random
changes in allele frequences due to genetic drift easily
override changes in allele frequences due to natural
selection, making the process of natural selection less
effective. Third, when a large population becomes frag-
mented into smaller populations, different alleles may
get lost or fi xed in different population fragments,
resulting in random genetic differentiation between
habitat fragments. This contrasts with adaptive genetic
differentiation as a result of a response to natural selec-
tion imposed by local selection pressures that underlies
local adaptation (section 7.2.1). This may be one of the
reasons that local adaptation is much more commonly
observed in large than in small populations (Leimu &
Fischer 2008), suggesting a lower ability to respond
to environmental variation and change through adap-
tive genetic differentiation in small than in large
populations.
Loss of heterozygosity and genetic diversity due to
inbreeding and genetic drift in small populations gen-
erally results in a reduction of average fi tness in the
population (Reed & Frankham 2003). For instance, in
the rare marsh gentian (
Gentiana pneumonanthe)
, size
and reproduction of plants strongly increases with the
number of loci at which they are heterozygous (Oost-
ermeijer
et al
. 1995). A meta-analysis of plant studies
by Leimu
et al
. (2006) shows that genetic variation -
measured as expected heterozygosity, number of alleles
or proportion of polymorphic loci (see Box 7.1) -
increases with population size and is signifi cantly and
positively related to fi tness. Since molecular variation
at neutral loci will generally not be strongly related to
the quantitative variation that is important for adapta-
tion and the ability to evolve (Reed & Frankham 2001),
one might expect that measures of genetic variation
based on near-neutral DNA markers are less strongly
related to fi tness than measures based on protein or
phenotypic markers. This is not born out by data;
according to the meta-analysis by Leimu and cowork-
ers the associations are at least as strong for DNA as for
allozyme data. Of course, associations between popula-
tion size and fi tness not only are mediated by genetic
factors, but also may in part be due to nongenetic
causes such as reduced quality of small habitat frag-
ments, and these factors may interact in their effects
on population fi tness.
7.2.4 Genetic rescue and genetic
restoration
Due to inbreeding and genetic drift, alleles with slightly
detrimental effects accumulate in small populations
(called the 'genetic load' of a population), causing low
mean population fi tness.
Genetic rescue
is the
intended introduction of (unrelated) individuals from
other populations to reduce the genetic load, that has
been advocated as a new management strategy to help
avoid extinctions of such low-fi tness populations
(Tallmon
et al
. 2004; Edmands 2007). Even if one had
wished to do so, restoration programmes that involve
reinforcement or reintroduction of populations often
cannot use source material from the exact local popu-
lations. Hence it introduces 'foreign' material, result-
ing in outcrossing with plants from different, genetically
diverged populations, a process called
admixture
.
Genetic rescue can be viewed as fi nding a balance
between the positive effects of admixture (heterosis
and increased genetic variation) and its negative effects
(outbreeding depression).
Heterosis
(or hybrid vigour) is the increased repro-
ductive fi tness of offspring resulting from admixture.
In section 7.2.2 we have seen that this can be based
either on dominance or overdominance of favourable
alleles. Most of the experimental studies in plants
indeed show increased fi tness of interpopulation
crosses, indicating potential for genetic rescue, includ-
ing the rare yellow pitcher plant (
Sarracenia fl ava
)
(Sheridan & Karowe 2000 ), the fi eld mustard (
Brassica
campestris
) (Newman & Tallmon 2001) and the rare
Bavarian scurvy - grass (
Cochlearia bavarica
) (Paschke
et al
. 2002). The second potential advantage of admix-
ture is increased genetic variation and the production
of novel genotypes, increasing the adaptive potential
of populations and their ability to cope with future
environmental change. Conversely, there are risks of
admixture if populations are locally adapted. The fi rst
risk is that it will dilute the locally adapted gene pool
(i.e. reduce the pool of local alleles whose additive
effects increase fi tness); this process is called
extrinsic
outbreeding depression
. The second risk is that admixture
may lead to the breakdown of co-adapted gene com-
plexes (positive interactions between alleles at different
loci) that may have evolved in the local population;
